P-5
A standard mixture for assessing adsorption performances of SPME fibers as a
function of extraction number. Since its introduction for the analysis of water pollutants, solid-phase
microextraction (SPME) has become increasingly popular for a wide range of fast
flavour analysis applications. Experience with the SPME technique for the
analysis of volatiles from alcoholic beverages has identified a number of severe
problems. These include low repeatability, artefact formation, competitive
adsorption and variable sensitivity. This paper describes the use of a standard
mixture to assess the variability in responses of the CARBOXEN-PDMS fibers with
varying numbers of samplings or varying heat treatments. The standard mixture
included volatiles typically found in foods and beverages (alcohols, esters,
acids, lactones). Volatiles from the head-space were sampled and analysed by
GC-FID under standardized conditions. The overall adsorption capacity of the
CARBOXEN-PDMS fiber decreases with increasing extraction number. It should be
noted that the adsorption of volatiles showed two distinct profiles, depending
on the compound volatility and polarity. Highly volatile compounds adsorption
capacity decreased whereas the adsorption capacity for the less volatile
compounds was similar or increased. A comparison of the amount of adsorbed
volatiles for a standard mix dilution series and for variable sampling times
clearly indicated that the reduction of sensitivity was attributable to a loss
of accessible coating surface for adsorption. The difference in behaviour
between the highly and lesser volatile compounds suggested irreversible changes
in the Carboxen coating, while the PDMS coating characteristics remained
unaffected. Exposure of the fibers to 260 C for an extended period did not
affect the adsorption characteristics of the fiber. The coating surface was
characterized by optical and scanning electron microscopy. These results
indicated that several causes may be responsible for the loss of adsorption
sites such as the irreversible binding of volatiles, the presence of adsorbed
particulate matter and the physical wearing of the coating. Using the standard
mixture, it is possible to adjust the experimental conditions to perform
quantitative analysis of food and beverage volatiles using fibers with a large
difference in their extraction number.
Jean-Pierre Dufour. Studies: MSc., 1975-PhD., 1979 (Louvain). Research
fellow (Johns Hopkins University, School of Medicine, Baltimore, MD) 1979-1981.
Appointments: Catholic University of Louvain, Professor 1981-1993 (Head of the
Department of Brewery and Food Industries, 1987-1993); Universidade Catolica
Portuguesa, Escola superior de Biotechnologia (Porto, Portugal), visiting
Professor 1989-1994; University Senghor (Alexandria, Egypt), associated
Professor 1992-1995; Expert for EEC and UNIDO (Implementation of sorghum malt
for the production of lager beer in Africa) (1994-1996); University of Otago,
Dunedin, New Zealand, Professor (1995-present), Chairperson and Head of
Department of Food Science. Expertise: Flavour science, fermentation science and
technology, malting and brewing sciences, yeast biochemistry/enzymology. Active
member of EBC Brewing Science Group, ASBC, IOB, Institute of Food Technology,
American Chemical Society, New Zealand Institute of Food Science and Technology.
Vice-president and Fellow of the New Zealand Institute of Food Science and
Technology, New Zealand delegate to IUFoST.
JEAN-PIERRE DUFOUR (1), Alexis Klein (2), Michelle Leus (1), and Pat Silcock
(1). (1) Department of Food Science, University of Otago, PO Box 56, Dunedin,
New Zealand, and (2) ENSBANA, Dijon, France.